Sound legit? Not really, although that's probably the EASIEST thing for him to do... if he can find a hammer.

The Springfield P9 is really a "sort of" clone of the CZ design, built around Tanfoglio-made components, which are subtly different in a lot of ways from the CZ. If it weren't for that, I'd suggest some of the parts offered by the CZ Custom Shop. The Custom Shop might know if their parts are compatible, however, so a call or email there might not be a bad idea.

Little things like the width of the hammer are different, and they changed over time, too, so that a part that might fit an early model might not fit one made a couple of years later. (I found that this could be true of some CZ parts, too.)

I don't there's much in the way of after-market parts for these older, small-framed Tanfoglio guns, so finding a better hammer will be difficult -- and the gunsmith will probably still have to polish and tune the new hammer.

You might find that just going to a lighter hammer spring might do what you want, and that's something you COULD do yourself. You can get those from Wolff Springs. http://www.gunsprings.com Once you're on the site, do a SEARCH for "Trigger Job."